Many containers, such as plastic bottles and metal cans, have heat shrinkable labels attached to them these days. Such heat shrinkable labels are prepared by printing images on a heat shrinkable multilayer film made of a thermoplastic resin.
Heat shrinkable multilayer films are required to have various properties including heat resistance, solvent resistance, and tearing properties along the perforation, as well as low-temperature shrinkability.
Materials used for heat shrinkable labels include polystyrene, polyester, and polyolefin.
More and more used plastic bottles collected for recycling are being reprocessed into flakes or pellets. Heat shrinkable labels are perforated in advance to make it easy for consumers to dispose of plastic bottles and heat shrinkable labels separately. Still, many plastic bottles are disposed of with the heat shrinkable labels on.
In such a case, collected plastic bottles are ground into a size of about several millimeters square to 10 mm square. Ground pieces of heat shrinkable labels having a density of lower than 1,000 kg/m3 and caps are removed by a gravity separator, and ground pieces of heat shrinkable labels having a density of 1,000 kg/m3 or higher are removed with an air separator. Desired recycled plastic flakes and recycled plastic pellets are obtained from these ground pieces of plastic bottles cleared of impurities.
The gravity separator is a device that puts the ground pieces in water to separate them into those that float on water (e.g., ground pieces of heat shrinkable labels having a density of lower than 1,000 kg/m3 and caps) and those that sink in water (ground pieces of heat shrinkable labels having density of 1,000 kg/m3 or higher and ground pieces of plastic bottles). The air separator is a device that scatters the ground pieces and provides air from below to blow away ground pieces of heat shrinkable labels. Because of their principles, the gravity separator has high processability per unit time and the air separator has low processability. Thus, a heat shrinkable label having a density of lower than 1,000 kg/m3, which is removable with a gravity separator, is demanded.
Unfortunately, heat shrinkable labels formed from polystyrene or polyester have a density of higher than 1,000 kg/m3, so that they cannot be separated with a gravity separator in the recycling process.
Patent Literature 1 discloses a heat shrinkable label that has an interlayer containing a cyclic olefin resin. Such heat shrinkable labels formed from polyolefin have a density of lower than 1,000 kg/m3, but have low heat shrinkage. They thus do not shrink sufficiently when attached to a container such as a plastic bottle, leading to poor finished quality. In addition, heat shrinkable labels formed from polyolefin have low rigidity. They are thus less suitable for automatic attachment machines and tend to cause problems such as label jam.
In view of the situation, there is a demand for a heat shrinkable label that has excellent shrinkability and rigidity while achieving low density.